This invention relates to high-voltage electrical transmission systems, and more particularly, to an electrical bushing through which an electrical conductor is brought into an electrical apparatus.
In construction of systems for the transmission of electric power, provision is made for connection of electrical conductors to equipment encased in metallic housings. Such a conductor is brought through an aperture in the housing via an insulating bushing. Due to the high voltage involved in transmission of electric power, which voltage can be on the order of hundreds of thousands of volts, special construction features are employed in fabricating the bushing.
Pressurized gas, particularly sulfur hexafluoride, SF.sub.6, has been used to provide the insulation. Also laminates of paper and metallic foil have been employed to provide a more uniform distribution of the electric field, the uniform distribution permitting a smaller physical size to the bushing while preventing arcing of electricity. The pressurized gas is not used in bushings for service above a few hundred thousand volts, above the 345 kV class, because the required gas pressure is so great as to pose a hazard to personnel and equipment. In the laminated structure, the paper is usually impregnated with some material such as epoxy resin or mineral oil to increase the dielectric strength of the structure.
The resin and the oil introduce further problems. In the case of the oil in the bushing, there is the threat of contamination of the SF.sub.6 used as a dielectric in the equipment being entered by the bushing, and resultant fouling of the filtration system used in removal of moisture and byproducts of the gas. The resin presents manufacturing problems in that, due to high viscosity of the resin, it is difficult to impregnate completely the paper with the resin. Also, large resin castings tend to develop cracks during curing of the resin. Thus, the resin is limited to bushings of smaller physical size than that which would be used above the 200-300 kV range. An exemplary form of the laminate structure with resin interposed between cylindrical metallic sheets is disclosed in the U.S. Pat. No. 3,394,455 of Grimmer.